Rotatable links, such as ball bearings, are known. A special type of ball bearing has an applied tension between the balls in the link and the face on which they move during rotation. This applied tension results in an increased friction in the movement of the balls, and the link may be characterized as sluggish in the rotary movement. However, the friction decreases with use of the link so that the sluggishness is reduced, and frequent adjustments of the applied tension are required to maintain the sluggishness.